Field of the Invention
The invention relates to a method and apparatus for determining ambient light and surface spectral reflectance from light reflected by a surface.
When light from an illuminating light source impinges upon a surface, a reflected beam of light is produced. The illuminating light has a characteristic spectral power distribution and the surface has a characteristic spectral reflectance. The light reflected by the surface has a spectral content determined by the spectral power distribution of the illuminating light and the spectral reflectance of the surface.
The human eye and brain have the ability to determine the colour of the surface merely by viewing the light reflected from it. In fact, this perceptual ability is such that the colour of the surface can be determined under a variety of lighting conditions. The human eye perceives the surface as the same colour under different ambient lighting conditions. In the fields of computer vision, colour television, photography, etc. it is desirable to produce this same perceptual ability in non-human devices.
U.S. Pat. No. 4,648,051 to Wandell et al. discloses a system which includes a number of sensors and a computer programmed to analyze light reflected by a surface. The computer produces approximations of the illuminating light and of the surface spectral reflectance of the surface. Such approximations involve a number of degrees of freedom or variables to describe the illuminating light or surface reflectance. Wandell requires that there be at least as many sensor classes as there are degrees of freedom in the ambient light approximation and requires that there be more classes of sensors than degrees of freedom in the surface reflectance approximation. In addition, the Wandell device requires that the illuminating light be constant everywhere on the surface, requires viewing a plurality of locations on the surface and requires that there be a significant variation of the colour of the surface within a small region thereon.
The invention disclosed herein views the surface at only one point to determine an estimate of surface spectral reflectance for that point and to determine an estimate of the illuminating light impinging upon the point. Thus, the surface spectral reflectance of each point on the surface can be determined individually and because each point is viewed individually, the apparatus does not require that there be a significant variation in the surface colour within a small region on the surface and does not require that the illumination of the surface remain generally constant over areas of the surface.